Metamorphism of the Kangâmiut dykes and the metamorphic and structural evolution of the southern Nagssugtoqidian boundary in the Itivdleq – Ikertôq region, West Greenland

Abstract

The structural evolution of the Nagssugtoqidian mobile belt is characterised by reworking of pre-existing Archaean rocks. At the southern Nagssugtoqidian boundary swarms of basic dykes, the Kangamiut dykes, intruded after the earliest Nagssugtoqidian movements. These dykes aet as time markers separating tectono-metamorphic events and record degree and extent of post-dyke metamorphism and deformation. The structural and metamorphic evolution at the southern Nagssugtoqidian boundary ean be summarised as follows. The rocks now exposed were stabilised in granulite and amphibolite facies in Archaean times. Uplift brought the rocks under low amphibolite facies conditions. Subsequent deformation (Nag. 1) caused the retrogression to low amphibolite facies and transformed the rocks into strongly schistose rocks along duetile, transcurrent, E-W trending shear zones. The Nag. 1 deformation was followed by intrusion of the mainly NE trending Kangamiut dykes. Later Nagssugtoqidian deformation (Nag. 2) affected country rocks with Nag. 1 fabrics as well as dykes and probably also Archaean gneisses unaffected by Nag. 1 deformation. In areas with intense Nag. 2 movements the deformation was characterised by duetile overthrusting towards the SSE along a linear zone striking ENE. New Nag. 2 fabrics were imposed on the country gneisses and dykes were transformed into strongly deformed amphibolites. The duetile overthrusting brought granulite facies rocks into juxtaposition with amphibolite facies rocks so that across the areas affected by Nag. 2 deformation a prograde metamorphic sequence with facies boundaries parallel to the overthrusting zone was established.